1. Field of the Invention
The present invention relates to an image forming apparatus and a power source that outputs a high voltage for image formation.
2. Description of the Related Art
An image forming apparatus will be described by using a printer as an example. The printer conventionally includes a mechanism illustrated in FIG. 12. In FIG. 12, the printer includes the following units. A photosensitive drum 101 is an image bearing member. A semiconductor laser 102 is a light source. A rotational polygonal mirror 103 is rotated by a scanner motor 104. A laser beam 105 is emitted from the semiconductor laser 102 to scan the photosensitive drum 101.
A charging roller 106 is configured to uniformly charge the photosensitive drum 101. A developing device 107 is a developer to develop an electrostatic latent image formed on the photosensitive drum 101 by toner. A transfer roller 108 is configured to transfer a toner image developed by the developing device 107 to recording paper. A fixing roller 109 is configured to fix the toner image transferred to the recording paper by heat.
A cassette feeding roller 110 feeds a sheet from a cassette which identifies a size of the recording paper to a conveyance path by rotating one round. A manual feeding roller 111 feeds a sheet from a manual feeding port which does not identify the size of the recording paper to the conveyance path. An optional cassette feeding roller 112 feeds a sheet from a detachable cassette which identifies the size of the recording paper to the conveyance path. An envelope feeder feeding roller 113 feeds sheets one by one from a detachable envelope feeder on which only envelopes can be loaded to the conveyance path. Conveyance rollers 114 and 115 are configured to convey sheets fed from the cassette.
A pre-feed sensor 116 detects a leading edge and a trailing edge of a sheet fed from other than the envelope feeder. A pre-transfer roller 117 feeds the conveyed sheet to the photosensitive drum 101. A top sensor 118 synchronizes image drawing (recording/printing) to the photosensitive drum 101 with sheet conveyance for the fed sheet and to measure a length of the fed sheet in a conveying direction. A sheet discharge sensor 119 detects presence or absence of a sheet after fixing. A discharge roller 120 discharges the sheet after fixing out of the apparatus.
A flapper 121 switches a conveyance destination (to out of the apparatus or to detachable two-sided unit) of a printed sheet. A conveyance roller 122 conveys a sheet conveyed to the two-sided unit to a reversing unit. A reversing sensor 123 detects a leading edge or a trailing edge of the sheet conveyed to the reversing unit. A reversing roller 124 reversed the sheet and convey the sheet to a re-feeding unit by sequentially rotating forward and backward. A re-feeding sensor 125 detects presence or absence of a sheet of the re-feeding unit. A re-feeding roller 126 feeds the sheet of the re-feeding unit again to the conveyance path.
FIG. 13 is a block diagram illustrating a circuit structure of a control system for controlling such mechanical units. In FIG. 13, a printer controller 1201 rasterizes image code data transmitted from an external device such as a host computer (not illustrated) into bit data necessary for printing in the printer, and reads and displays printer internal information. A printer engine control unit 1202 controls an operation of each unit of a printer engine according to an instruction from the printer controller 1201, and notifies the printer controller 1201 of the printer internal information.
A sheet conveyance control unit 1203 drives or stops a motor or a roller for conveying the recording paper according to an instruction from the printer engine control unit 1202. A high voltage control unit 1204 performs output control of high voltage in each process such as charging, developing and transfer according to the instruction from the printer engine control unit 1202.
An optical system control unit 1205 controls driving or stopping of the scanner motor 104 and lighting of a laser beam according to the instruction from the printer engine control unit 1202. A fixing temperature control unit 1207 adjusts a temperature of a fixing device to a temperature instructed by the printer engine control unit 1202.
An optional cassette control unit 1208 drives or stops a driving system according to the instruction from the printer engine control unit 1202, and notifies the printer engine control unit 1202 of a paper presence state and paper size information.
A detachable two-sided unit control unit 1209 performs sheet reversing and a re-feeding operation according to the instruction from the printer engine control unit 1202, and notifies the printer engine control unit 1202 of operation states thereof at the same time.
An envelope feeder control unit 1210 drives or stops the driving system according to the instruction from the printer engine control unit 1202, and notifies the printer engine control unit 1202 of a paper presence state.
As a high voltage output value, there is a voltage (hereinafter, referred to as a bias) for which a predetermined voltage difference is correlatively required for individual outputs. Examples are outputs of a charging direct current (DC) voltage and a developing DC voltage. A difference between these two bias values affects an image density (contrast).
FIG. 14 illustrates schematic configurations of charging and developing DC bias application circuits 701 and 801. The charging DC bias application circuit unit 701 includes a voltage setting circuit unit 702 which can change a set value according to a pulse width modulation (PWM) signal, a transformer driving circuit unit 703, a high voltage transformer 704, and a feedback circuit unit 705. The feedback circuit unit 705 detects a voltage value applied to a load by a resistance R71, and transmits the detected voltage value as an analog value to the voltage setting circuit unit. Based on this value, control is performed so as to apply a fixed voltage.
The developing DC bias application circuit unit 801 includes a voltage setting circuit unit 802 which can change a set value according to a PWM signal, a transformer driving circuit unit 803, a high voltage transformer 804, and a feedback circuit unit 805. The feedback circuit unit 805 detects a voltage value applied to a load by a resistance R81, and transmits the detected voltage value as an analog value to the voltage setting circuit unit. Based on this value, control is performed so as to apply a fixed voltage.
With this configuration, constant voltage values can be applied at the charging DC bias application circuit unit and the developing DC bias application circuit unit by performing a series of control operations. Apparatuses with such configurations are discussed in Japanese Patent Application Laid-Open Nos. 2006-162893 and 6-3932.
In the DC bias circuit structure, each voltage value is controlled constant. By improving accuracy of an output voltage at each circuit, accuracy of a difference (e.g., contrast voltage) in voltage values between the biases is improved.
Increasing print speeds has been accompanied by an image problem such as density variance at conventional high voltage accuracy. In other words, the apparatus is operated at higher speed so as to increase the number of prints (number of formed images) per unit time, and hence voltage control for correcting an image density may not be in time. In the case of achieving higher image quality, the conventional high voltage circuit structure cannot sufficiently correct variance on voltage accuracy for correcting image density variance in a page or between pages. To realize control with a higher voltage accuracy, shift to a control method is required which does not control each bias variance but controls an output voltage in association between biases.
These problems will be described below more in detail. As examples, FIGS. 15A and 15B illustrate a potential (Vd) of a charging DC bias and a potential (Vdc) of a developing DC bias. The photosensitive drum is set to a potential VL after laser irradiation. In the current circuit structure in FIG. 15A, the potential Vd changes to cause a change in potential difference between Vdc and Vd, and a margin to an image failure (referred to as a fogged image) where an image is unnecessarily developed is reduced. A potential difference between VL and Vdc also changes and causes a reduction in a margin before image density unevenness occurs. However, as illustrated in FIG. 15B, if output control associating biases with each other is performed, even when the potential Vd changes, the potential difference between Vdc and Vd is kept constant and the potential difference between VL and Vdc is kept constant.
In the developing processing, an electrostatic adsorption power applied to toner depends on the potential difference between VL and Vdc. Thus, when the potential difference between VL and Vdc is constant, a force applied to the toner is constant, and a density of toner adsorbed on the photosensitive drum is constant. Thus, a margin to a fogged image or image density unevenness may not reduce.
As other examples, FIGS. 16A and 16B illustrate a potential (Vdc) of a developing DC bias and a potential (Vrb) of a developing blade bias. The developing blade bias is provided for the purpose of charging charges of toner itself close to a developing DC bias value, and it is necessary to be set close to a developing DC bias output. However, when a developing blade bias is output at a potential equal to or a plus side of a developing DC bias, toner is fixed to the developing blade to cause an image failure. Thus, a predetermined minus potential difference with respect to the developing DC bias is necessary for the developing blade bias.
In the current circuit structure in FIG. 16A, the potential Vdc changes to cause a change in potential difference between Vdc and Vrb, and a margin to charging of toner and a margin to toner fixing are reduced. However, as illustrated in FIG. 16B, when output control associating biases with each other is performed, even if the potential Vdc changes, the potential difference between Vdc and Vrb is kept constant, and the margin to the potential for charging the toner does not reduce.